Vacuum brazing of aluminum alloy workpieces

ABSTRACT

An improved practice for the vacuum brazing of aluminum alloy workpieces in a multi-chamber braze furnace is disclosed. When magnesium-containing aluminum alloys are brazed in a furnace having a braze chamber and adjacent exit chamber, there may be advantage in providing thermal barriers between a load of hot brazed workpieces and the furnace walls of the exit chamber.

This is a division of application Ser. No. 937,888, filed on Dec. 4,1986 pending.

This invention relates to the vacuum brazing of magnesium-containingaluminum alloy workpieces. More specifically, this invention relates toimprovements in practices for the large scale vacuum brazing of suchaluminum workpieces in brazing furnaces.

BACKGROUND OF THE INVENTION

The manufacture of aluminum heat exchangers such as air conditionerevaporator cores and radiators is a sophisticated technology in that itrequires the simultaneous formation of many brazed joints between thealuminum alloy tubes, fins and headers or tanks. In order to supplymolten braze alloy at the intended joints, at least some of suchcomponents, for example, the tubes, are formed of two or more aluminumalloy layers in a sandwich-type sheet construction. In the sandwich, theprincipal layer or base layer is an aluminum alloy in sheet formselected for its strength and corrosion resistance requirements. Rollbonded to one or both surfaces of such base layer is a suitable aluminumbrazing alloy composition. The aluminum braze alloy compositioncomprises aluminum with an appreciable amount of silicon such that thecomposition will melt and flow on the base layer at a brazingtemperature below the melting point of the base layer.

In the production of such aluminum heat exchangers, aluminum alloy sheetmaterials are shaped and assembled together to form the desired part.The assembly must then be heated in a suitable environment to melt thebraze alloy layers, cause them to flow and wet underlying and adjacentsurfaces and form brazing fillets at many joints at the same time. Insome applications, such brazing operations have been carried out invacuum furnaces. In this practice, magnesium in suitable quantities isincorporated into either the base aluminum alloy composition, the brazealloy composition or both. When the assembled aluminum sheet componentsare heated in vacuum to a braze temperature, the magnesium vaporizes orsublimes to getter the environment in the vicinity of the molten alloyto facilitate wetting of the aluminum surfaces to be joined. Thispractice is disclosed, for example, in U.S. Pat. Nos. 3,321,828;3,322,517; 3,373,482 and 3,373,483.

Where large numbers of aluminum workpieces are to be brazed in acontinuous process, it is generally preferable to employ a multi-chambervacuum brazing furnace. In the operation of such a continuouslyoperating multi-chamber furnace, a plurality of workpieces aresequentially advanced into the inlet of the furnace through therespective isolatable operating chambers and the brazed products removedfrom the furnace outlet.

A suitable furnace might comprise, for example, three stages orchambers. The furnace may be aligned with the chambers in a straightflow path. The inlet and outlet for each chamber can be closed off withsuitable doors so that the temperature, pressure and other environmentalparameters therein may be controlled. In a typical flow-through vacuumbrazing furnace, a group of assembled heat exchangers is firstintroduced into a preheat chamber where the absolute pressure is reducedto about 5×10⁻³ torr and the workpieces are heated by use of radiantheating elements to a temperature of about 800° F. The preheatedworkpieces are then transported to the next chamber where the brazeoperation takes place. The pressure is reduced to a level of 10⁻⁵ to10⁻⁶ torr. The temperature is increased to about 1100° F. such that thebraze alloy cladding on surfaces of at least some of the workpiececomponents melts and flows under capillary action to the joints to beformed. At the high temperatures and low pressures of the braze chamber,magnesium vapor evolves from the alloy to react with residual oxidizingconstituents in the chamber that could inhibit wetting of the workpiecesby the molten braze alloy and the formation of a braze joint. Thestill-hot workpieces are then moved into the third chamber which servesas an exiting vestibule. The exit chamber is initially evacuated to alow pressure suitably 5×10⁻³ torr. As soon as the workpieces haveentered the chamber and it has been isolated from the braze chamber, dryair is admitted to effect some cooling of the workpieces and to raisethe pressure in the chamber to that of the atmosphere. The outlet doorof this chamber is then opened and the workpieces are removed from thefurnaces into ambient air for further cooling.

In the prolonged operation of continuous multi-chamber vacuum brazingfurnaces of the type described, problems have arisen that were notpredictable from batch-type or other multi-chamber brazing operations.Successful continuous operation of these flow-through furnaces dependsupon repeatedly obtaining the desired operating conditions in eachchamber. Some flow-through furnaces are relatively large, havingchambers, for example, that may be 6 feet wide by 10 feet high by 20feet long. More than 100 automotive air conditioner evaporators may betreated at the same time in each of the three chambers. Magnesium thatis evolved from the workpieces in the braze and exit chambers tends tocollect as magnesium or magnesium oxide on the furnace wall surfaces.The furnaces must be cleaned from time to time to remove this oxide. Insome such brazing lines, continuous operations have been interrupted bythe occurrence of excessively high pressures in the exit and brazesections as the workpieces are shuttled into the exit chamber. The highpressures in the braze section overwhelm the vacuum pumps and causeshutdown of the braze line.

It is an object of the present invention to provide an improved practicefor the operation of such multi-chamber aluminum vacuum brazing furnacesto prevent such high pressure events in the braze chamber and avoidinterruptions of the desired continuous brazing operations. It isanother object of the present invention to provide an improvement in thedesign and construction of the exit chamber of such vacuum brazingfurnaces for more efficient removal of accumulated magnesium oxidedeposits and for avoiding high pressure impulses that upset operationsin the braze chamber of the furnace.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of my invention, these andother objects and advantages are accomplished by providing suitable heattransfer barriers in the exit chamber of the furnace positioned betweenthe bundle of hot brazed workpieces and the relatively cold walls of theexit chamber of the vacuum furnace. The use of such barriers is based onthe following discovery.

I have discovered that in the normal operation of aluminum vacuumbrazing furnaces of the type described above, magnesium oxide particlesaccumulate on the walls of the exit chamber of the furnace. These wallsare typically maintained at temperatures of about 150° F. to 170° F.They provide a natural surface for deposit of magnesium evolved from thebrazed workpieces or the magnesium oxide particles formed as aconsequence of the magnesium reacting with oxygen, water or otherresidual oxidizing constituents in the chamber. The buildup of magnesiumoxide on the furnace walls continues as the furnace operates. The rateof buildup is a function of the number of workpieces being brazed andthe resulting total surface area--especially the area exposed to thecool wall surfaces.

In addition to magnesium oxide buildup on the furnace exit chamberwalls, moisture is adsorbed on the magnesium oxide particles and walls.Moisture is introduced into the exit chamber when the exit door isopened to remove the workpieces from the furnace. After there has beensufficient buildup of magnesium oxide and adsorbed moisture on the wallsof the exit chamber, difficulties may then be experienced in the brazechamber. If the furnace is otherwise functioning properly, I believethat the high pressure events interrupting brazing operations areinitiated when a load of hot brazed workpieces is introduced into theexit chamber. Despite the low pressure in the chamber, some moistureremains on the walls of the furnace. Heat radiated from the brazedworkpieces to the cold walls drives moisture into the chamber where itreacts with magnesium still evolving from the workpieces. The reactionof the magnesium with the moisture produces hydrogen or other gases thatincrease the pressure in the exit chamber. While the door between thebraze chamber and the exit chamber is opened, the gases enter the brazechamber, adversely affecting pressure control in that critical part ofthe furnace.

In accordance with my invention, I interpose heat shields between thehot workpieces in the exit chamber and the relatively cold walls. Theseheat shields may suitably be low carbon steel sheets or strips that hangin the radiation path between the workpieces and the furnace walls.Thus, these heat shields are the first furnace surfaces to be heatedrather than the walls. Preferably, they are of a solid material havingsufficient mass to hold their temperature below about 900° F. but suchthat the temperature may rise high enough to provide radiant heattransfer to the furnace walls. Moreover, magnesium and magnesium oxideparticles deposit on these heat shields. Since the walls are notdirectly heated by the hot workpieces, the moisture is not immediatelydriven from them. Before heat is reradiated from the heat shields to thewalls, the braze chamber can be sealed from the exit chamber and thevacuum system of the exit chamber can then remove any moisture or othergases that eventually result from the presence of the hot workpieces.Thus, where the quantity and surface area of the workpieces being brazedare such that copious quantities of magnesium are evolved in the exitchamber, my heat shields serve as a buffer to intercept the flow ofmagnesium and magnesium oxide particles and the thermal radiation fromthe workpieces toward the walls of the furnace. The shields facilitatethe easier removal of magnesium oxide from the furnace and mitigatepressure buildup in the braze chamber.

Other objects and advantages of my invention will become more apparentfrom a detailed description which follows. In this description,reference will be had to the drawings in which:

FIG. 1 is a schematic plan view of a flow-through three chamber vacuumbrazing furnace and related material handling and processing equipment;

FIG. 2 is a cross sectional view of the exit chamber of the furnaceillustrating the heat shields therein; and

FIG. 3 is a longitudinal elevation view of the furnace partly brokenaway to illustrate the heat shield and other interior furnace parts.

DETAILED DESCRIPTION OF THE INVENTION

The practice of my invention will be described in connection with thebrazing of aluminum automotive air conditioner evaporators. By way ofexample, each evaporator is formed by assembling alternate layers oftube-defining sheet alloy and fin stock sheets. The alternate fins andtubes are suitably banded together to form a heat exchanger body forbrazing. The outer bounds of the evaporator assembly generally define arectangular box, e.g., approximately 13 inches long, 8 to 9 inches highand 3 inches wide.

Further by way of example, the tube sheets are formed of a compositebraze alloy having a base or core portion of 3005 aluminum alloy andsurface layers of 4047 alloy clad thereto by roll bonding. The corealloy nominally contains by weight 0.5 percent magnesium, b 1.2 percentmanganese and the balance aluminum. The composition of this alloy isselected for purposes of strength and corrosion resistance of the heatexchanger. The braze alloy has a nominal composition by weight of 12.5percent silicon, about 0.15 percent magnesium and the balance aluminum.Each clad braze alloy layer constitutes about 10 percent of thethickness of the composite sheet. The composition of the braze alloy isadjusted so it will melt at a temperature range (e.g., 1070°-1080° F.)suitably below the melting temperature of the core alloy. The fin stockalloy may also contain magnesium.

A plurality of assembled heat exchangers, for example, 120, are loadedinto a suitable carrier fixture in rows and columns so that the largestfaces are exposed. This carrier with the many workpieces for brazing istransported through the furnace as a unit. Continuous operation of athree-compartment in-line furnace to braze carrier loads of this sizeand arrangement has led to the brazing chamber pressure increaseproblems described above.

The practice of my invention will now be described by tracing the pathof a group of such exchangers through a three-chamber in-line vacuumbrazing furnace. Referring to FIG. 1, such a furnace is depicted inschematic plan view at 10. The furnace has a preheat chamber 12, a brazechamber 14, and an exit chamber 16. A carrier fixture containing heatexchangers is suspended from a conveyor 18 at a work station labeled"LOAD". The fixture is then moved to a cross transfer conveyor 20whereby it is aligned with the entrance door 22 of the preheat chamber12.

When the preheat chamber 12 is available upon removal of the previousgroup of assembled heat exchangers, it is backfilled with dry air (-45°F. dew point) through an air inlet (not depicted). Door 22 opens and theload of heat exchangers is moved into the preheat chamber 12. Such aload is depicted in schematic outline at 23'. By way of example, themajor dimensions of the preheat chamber which is generally oval in crosssection are approximately 6 feet wide by 10 feet high by 20 feet long.The load of heat exchangers to be brazed is about 18 feet long by 6 feethigh by 10 inches wide. At the commencement of the brazing operation,doors 22 and 24 are closed to isolate the preheat chamber 12 from thebraze chamber 14. Each furnace door opening is about 20 inches wide, andthey slide from a closed position to a retracted open position. As soonas preheat chamber 12 is closed, the pressure is reduced in the chamberand the assembly is preheated to about 800° F. by radiant heatingelements 25 on each side of the chamber 12. The pressure in the chamberis reduced through duct 29 by means of a rotary lobe blower 30 and twomechanical vacuum pumps 32 operating in parallel with each otherdownstream from the blower 30. The pressure in the preheat chamber 12 isreduced to about 5×10⁻³ torr By way of example, the cycle of moving agroup of heat exchangers into the preheat chamber, closing the chamber,reducing the pressure and heating the exchangers, and then moving theminto the braze chamber requires about 20 minutes.

It will be appreciated that the operation of this furnace 10 indicatedschematically in FIG. 1 is intended to be continuous, and therefore thecycle time in each chamber is the same. Also, each chamber issubstantially the same size.

As soon as a previous group of heat exchangers has been carried out ofthe braze chamber 14 and its exit door 26 has been closed, door 24between the preheat chamber 12 and the braze chamber 14 is opened andthe load of heat exchangers is moved into the braze chamber 14. Door 24is also closed. Under preferred conditions, the pressure in the brazechamber never exceeds 5×10⁻³ torr during operations. The pressure in thebraze chamber is then further reduced through ducts 33 by three 35-inchdiffusion pumps 34 backed up by mechanical vacuum pumps 36. The brazepressure is about 10⁻⁵ to 10⁻⁶ torr. The load of heat exchangers,indicated as 23" in the braze chamber 14, is further heated by radiantelements 28 on each side to a suitable braze temperature range, forexample, 1100°-1110° F. Under these conditions, braze alloy layers oneach of the heat exchanger assemblies melts and flows by capillaryaction to adjacent joints to be formed. This action is promoted by theevolution or sublimation of magnesium from the aluminum alloys. Aspreviously stated, the magnesium serves to further purge the brazingenvironment of oxidizing constituents which would inhibit effectivebrazing. The load of heat exchangers is then ready to be transferred toa cooler area so that the heat exchanger bodies can be cooled and thefillets solidified.

Before door 26 is opened, any preceding load is removed from the exitchamber 16. Its exit door 38 is then closed. The chamber 16 is evacuatedthrough duct 39 by means of a rotary lobe blower 40 and two mechanicalvacuum pumps 42 to a suitable low pressure of about 5×10⁻³ torr. Door 26is opened and the load of very hot heat exchanger assemblies enters theexit chamber 16 with the chamber at its reduced pressure. The load isdesignated as 23"' in exit chamber 16. The hot workpieces radiate heattoward the surrounding walls of the furnace and the assemblies continueto emit magnesium. My invention involves a method of modifying thetransfer of heat and the deposition of the magnesium in the exitchamber. This practice will be described in more detail below followingcompletion of the description of the overall brazing process.

After door 26 has been closed, isolating the braze chamber 14 from theexit chamber 16, dry air is admitted to the exit chamber from a sourceno shown and through a conduit not shown into the exit chamber 16 toprovide some cooling of the brazed workpieces and to raise the pressureof the chamber 16 so that the load 23"' of workpieces 21 (FIG. 2) can beremoved to the atmosphere. The dry air with a dew point of about -45°F., and heat radiated from the hot heat exchangers to the cold furnacewalls, lowers the temperature of the load of heat exchangers to about1000° F. Door 38 is then opened and the load moved to a cross transferconveyor 44, which is appropriately shielded to prevent operator contactwith the hot workpieces. The cross transfer conveyor 44 lines the loadup with transfer conveyor 18, which receives the load of hot workpiecesand transfers it to a cooling zone 46 at which outside air is blown onthe brazed workpieces to reduce the temperature to approximately ambienttemperature. The load is transferred to an unload station 48 at whichthey are removed from the conveyor 18.

As summarized above, there is difficulty in maintaining continuous andefficient operation of the furnace because of a sudden and excessiveincrease in pressure in the braze chamber 14 after door 26 is opened anda load 23 of heat exchangers is transferred into exit chamber 16. At thetime of this transfer, it will be recalled that the pressure in thebraze chamber 14 is initially about 10⁻⁵ to 10⁻⁶ torr and the pressurein the exit chamber 16 is about 5×10⁻³ torr. Normal design of thefurnace requires that the pressure in the braze chamber not greatlyexceed 5×10⁻³ torr so that the diffusion pumps 34 are not overloaded.However, it has been observed with a load of heat exchangers such asthat described above that after prolonged continuous operation of manydays, sudden pressure increases to 70×10⁻³ torr or more are experiencedwhich may cause a protective shut down of the diffusion units. It hasbeen determined that such pressure increase may occur even though thereare no furnace leaks and the vacuum systems are working properly. It hasbeen my observation that the pressure increase occurs after there hasbeen a buildup of magnesium or magnesium oxide particles on therelatively cold walls of the furnace (indicated at 50 in FIG. 2). Thewalls 50 of the whole furnace are water jacketed (not shown so that thewall temperature during operation does not exceed 150°-170° F. forreasons of operator safety and structural strength. When a carrier load23"' of hot brazed heat exchangers 21 enters the exit chamber 16,magnesium is still being vaporized. Much of it is believed to react withoxygen or water vapor in the chamber and form particles of magnesiumoxide which normally would be deposited on the relatively cold furnaceinner walls 50. When the carrier loads are removed from the furnace,ambient air enters and remains there momentarily during each brazingcycle. Some moisture from that air is adsorbed onto the relatively coolsurfaces of walls 50, even though they are at 150°-170° F. Although exitchamber 16 has been pumped down to a relatively low pressure when a loadenters the chamber, the radiated heat to furnace walls 50 drives offmoisture. The sudden increase of moisture in the chamber reacts withmagnesium evolving from the carrier load to form gases that flow backinto the braze chamber 14 while door 26 is opened.

Whatever the mechanism causing the increase in pressure, I have foundthat by placing sheets of plain carbon steel sheet between the carrierload 23"' and walls 50, the unwanted pressure fluctuations in chamber 14are avoided. Heat shields are shown at 52 in FIGS. 2 and 3. Even thoughmoisture accumulates on furnace walls 50, heat radiating from thecarrier load is first received by the heat shields 52. The overalldimensions of each shield are about 18 feet long by 9 feet high by 1/10inch thick. As best seen in FIG. 3, a shield is suitably formed of sixindividual sheets 56 of steel, each three feet wide by nine feet high.Each shield 52 may be supported on upper and lower rails carried onposts affixed to furnace wall 50. Such posts and rail are indicated at54. The sheets 56 may simply abut each other as shown to form the shield52.

Particles of magnesium and magnesium oxide that may be formed in thechamber are deposited on the heat shields 52. After door 26 is closedand as air is admitted to the chamber, the heat shields reradiate heatto the furnace walls 50 to promote vaporization of moisture from thewalls. However, by this time the exit chamber 16 is isolated from thebraze chamber 14 and the vacuum pumps remove the moisture and anygaseous reaction products of the vapor with magnesium. The mass of heatshields 52 are such that, in this application, they attain a maximumtemperature of about 800° F. Magnesium and magnesium oxide will depositon a surface at such temperature, and the shields are hot enough toslowly drive some moisture off walls 50 so that it can gradually bepumped out through duct 39.

Thus, I have found by employment of heat shields as indicated, reactionsin the exit chamber that would adversely affect the braze environment inthe braze chamber are avoided or delayed until the braze chamber door 26is closed. As shown schematically in FIG. 3, heat shields may be adaptedto accommodate conveyor shafts 58 and other necessary operatingcomponents of the exit chamber. In other words, the shields do not haveto completely block all heat transfer from the brazed assemblies to thefurnace walls. However, the shield must be of sufficient area and massto delay an evolution of pressure increasing gases until door 26 isclosed to isolate the braze chamber from the exit chamber. I have alsofound that the heat shields collect magnesium and magnesium oxideparticles. It is easier to remove the particles from the heat shieldsthan from the furnace walls. The shields may be of a solid material thatcan withstand the high temperature described above as well as thetemperature fluctuations encountered in furnace operation.

While my invention has been described in terms of specific embodimentsthereof, it will be appreciated that other forms could readily beadapted by those skilled in the art. Accordingly, the scope of myinvention is to be considered limited only by the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A furnace for vacuumbrazing magnesium containing aluminum alloy workpieces, said furnacecomprising a brazing chamber and an exit chamber, means for sequentiallytransporting the workpieces into and through said chambers and meanscomprising doors for isolating said chambers for each other and theambient when the workpieces are being treated therein, the brazingchamber being adapted to accommodate heating of such workpieces to abrazing temperature at a sufficiently low pressure for magnesiumgettered brazing to occur, the exit chamber comprising chamber definingwalls and being adapted to initially receive hot brazed workpiecestransported from the brazing chamber through a doorway between them atan exit chamber pressure compatible with the pressure in the brazingchamber, the exit chamber further comprising solid barriers to thermalradiation interposed between the hot workpieces and exit chamber walls.2. A furnace for vacuum brazing magnesium containing aluminum alloyworkpieces, said furnace comprising a brazing chamber and an exitchamber, means for continually and sequentially transporting theworkpieces into and through said chambers and means comprising doors forcyclically isolating said chambers from each other and the ambient whenthe workpieces are being treated therein, the brazing chamber beingadapted to accommodate heating of such workpieces to a brazingtemperature at a sufficiently low pressure for magnesium getteredbrazing to occur, the exit chamber comprising cooled chamber-definingwalls and being adapted to initially receive hot brazed workpiecestransported from the brazing chamber through a doorway between them atan exit chamber pressure compatible with the pressure in the brazingchamber, the exit chamber further comprising solid barriers to thermalradiation interposed between the hot workpieces and exit chamber walls,said barriers having sufficient mass to delay heating of the chamberwalls by each introduction of the hot workpieces until the braze chamberagain has been isolated from the exit chamber.